Why cement hardens. Setting time of cement mortar

Building materials appeared at the moment when, at the dawn of our civilization, the first people began building houses and fortifications. Over time, mankind has been looking for materials that have great strength and availability in any place of residence. After long searches and experiments, it was found that finely crushed limestone and gypsum, when mixed with water and minerals, acquires special astringent properties.

After hardening, it forms a monolithic joint having the characteristics of a hard stone. From that moment, cement began to be produced in large quantities and used in the construction of large and small structures. Passing once again past a building made of stone and metal, we often ask ourselves the question: “So how is cement made?”

Interesting fact: during the construction of the Egyptian pyramids, the pharaohs used a technology similar to the production of concrete. A mixture of crushed limestone and stone chips was poured with water and turned into monolithic stone blocks.

What is cement made from?

The first stage of production begins in a limestone quarry, when the components of the future cement are removed from the soil with the help of mining machines. In order for the building material to have the required strength, limestone is chosen for production, which lies close to the surface. In its composition, in large quantities, there is silicon, iron and aluminum oxide. If you dig deeper, the rock will be cleaner, but with a high content of calcium carbonate. The mined stone, if necessary, is sorted and sent to production, where the proportions are changed to obtain cement of different grades.

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limestone processing

At the plant for the production of cement, the rock is unloaded into the apparatus for the primary crushing of stones. Large boulders, under the influence of a pressing force of several tons, are gradually crushed to the size of a tennis ball and fed to the conveyor. Small and large stones are sent to secondary crushing, where they are reduced to the size of a golf ball and into a fine powder. Limestone, with different percentages of calcium carbonate, is processed separately.

Scheme of the line for crushing and drying limestone: 1 - belt feeder PL-650; 2 – magnetic separator; 3 – drying complex; 4 - elevator; 5 - supply hopper with a sliding head; 6 – belt feeder PL‑500; 7 – mill МЦВ-3; 8 – rotary jet mill MRS-2/770; 9 - cyclone-bunker TsB-4.5; 10 – dust collector II ПЦ-2.0 with a bunker; 11 – bag filter FRI-60; 12 – sector feeder PS-1V; 13 – VVD fan; 14 – medium pressure fan; 15 - slide gates; 16 - compressor.

This is necessary for their further mixing in different proportions and according to a certain technology in order to produce cement of different grades.

Sorting and grinding

Fine limestone, with the help of a sorting loader, is placed in dry warehouses, protected from moisture and temperature changes. Heaps are formed from the raw mix, of different composition, ready for the grinding stage. On the conveyor, the crushed stone enters the grinding machine - roller mill, in which limestone dust is formed.

When interacting with water, it hardens and turns into the so-called cement stone. However, few people know the essence of this process: how it hardens, why it hardens, what awareness of the ongoing reaction gives us and how we can influence it. At the moment, understanding of all stages of hydration allows scientists to invent new additives in concrete or cement, one way or another affecting the processes occurring during the setting of cement and hardening of a concrete or reinforced concrete structure.

In general, there are two main stages in the process of curing concrete:

• concrete setting a rather short stage occurring in the first day of the life of concrete. The setting time of concrete or cement mortar depends significantly on the ambient temperature. At the classical design temperature of 20 degrees, the cement begins to set approximately 2 hours after the cement mortar has been mixed, and the setting ends approximately three hours later. That is - the setting process takes only 1 hour. However, at a temperature of 0 degrees, this period stretches to 15-20 hours. What can I say, if the very beginning of cement setting at 0 degrees begins only 6-10 hours after the concrete mixture has been mixed. At high temperatures, for example, when steaming reinforced concrete products in special chambers, we accelerate the setting period of concrete up to 10-20 minutes!

  During the setting period, the concrete or cement mortar remains movable and can still be acted upon. This is where the thixotropy mechanism comes into play. While you "move" the concrete that has not set to the end, it does not go into the hardening stage, and the cement setting process is stretched. That is why the delivery of concrete on concrete mixers, accompanied by constant mixing of the concrete mixture, is able to preserve its basic properties. If you wish, read the details about the basic properties and composition of concrete.

  From personal experience, I can recall extraordinary cases when our mixers with concrete stood and "threshed" at the facility for 10-12 hours, waiting for unloading. Concrete in such a situation does not harden, but some irreversible processes occur that significantly reduce its quality in the future. We call it concrete welding. Such events are especially critical in the summer in the heat. Remember the shortened setting time of cement at high temperature, which we talked about above. Managers and dispatchers of the BESTO Company try to avoid such incidents, but sometimes unforeseen situations occur, mainly related to the collapse of low-quality formwork. Concrete is spilling, everyone is running around trying to collect it, restoring the formwork, and time goes by, and concrete mixers with concrete that have not yet been unloaded stand and thresh. Well, if there is where to redirect, but if not? In a word, trouble.

• hardening of concrete This process occurs immediately after the end of the setting of the cement. Imagine that we finally put the concrete into the formwork with the help of a concrete pump, it seized safely, and here the concrete hardening process actually begins. In general, the hardening of concrete and the curing of reinforced concrete products takes not a month, or two, but years. The 28-day period is regulated only in order to guarantee a certain brand of concrete for a given period. The graph of curing of concrete or reinforced concrete products is non-linear and in the first days and weeks the process is most dynamic. Why so? And just let's figure it out. It's time to talk about the process of cement hydration.

Mineralogical composition and hydration of cement

We will not analyze the stages of obtaining Portland cement here, for this there is a special section that describes the production of cement in more detail. We are only interested in the composition of cement and its main components that react with water when mixing cement mortar or concrete. So. Four minerals obtained as a result of all stages of cement production are considered as the basis of Portland cement:

• C3S tricalcium silicate
• C2S dicalcium silicate
• C3A tricalcium aluminate
• C4AF Tetracalcium aluminopherite

The behavior of each of them at different stages of concrete setting and hardening is significantly different. Some minerals react with mixing water immediately, others a little later, and still others - it's not at all clear why they "hang around" here. Let's look at them all in order:

C3S tricalcium silicate 3CaO x SiO2 a mineral involved in the process of increasing the strength of cement over time. Without a doubt, it is the main link, although, during the first days of the life of concrete, tricalcium silicate has a serious faster rival C3A, which we will mention later. The process of cement hydration is isothermal, that is, a chemical reaction accompanied by the release of heat. It is C3S that “heats” the cement mortar during mixing, stops heating from the beginning of mixing until the moment of setting, then heat is released during the entire setting period, and then a gradual decrease in temperature occurs.

Tricalcium silicate and its contribution to the strength development of concrete is most significant only in the first month of the life of a concrete or reinforced concrete structure. These are the same 28 days of normal hardening. Further, its influence on the set of strength of cement is significantly reduced.

C2S dicalcium silicate 2CaO x Si02 begins to act actively only a month after the cement has been mixed in the concrete mixture, as if taking a shift from its tricalcium silicate brother. During the first month of the life of concrete or concrete goods, he generally plays the fool and waits in the wings. This period of idleness and relaxation can be significantly reduced through the use of special additives in cement. But, its action lasts for years, during the entire period of increasing the strength of reinforced concrete, reinforced concrete or concrete.

C3A tricalcium aluminate 3CaO x Al2O3 the most active of the above. He begins vigorous activity from the very beginning of the process of grasping. It is to him that we owe for the set of strength, during the first days of the life of concrete or reinforced concrete. In the future, its role in hardening and curing is minimal, but in speed it has no equal. You can’t call him a marathon runner, but perhaps a sprinter, yes.

C4AF Tetracalcium aluminopherite 4CaO x Al2O3 x Fe2O3 this is just the one that - "it is not clear why he is hanging around here at all." Its role in the set of strength and hardening is minimal. A slight effect on the set of strength is noted only at the very latest stages of hardening.

All of these components, when mixed with water, enter into a chemical reaction, due to which there is an increase, adhesion and precipitation of crystals of hydrated compounds. In fact, hydration can also be called crystallization. So it's probably clearer.

The BESTO company supplies ready-mixed concrete and mortar, made with the use of the most modern additives, which make it possible to obtain concrete mixtures and cement mortars with improved frost resistance, water resistance, mobility, etc. Modern dosing and concrete mixing equipment helps to achieve the best results in terms of the uniformity of the composition of the concrete mixture or cement mortar.

I hope I didn't hydrate your brains with my silicates and aluminates. With tricalcium greetings, Eduard Minaev.

At all times, people have been building for their own needs, starting with ancient buildings and ending with modern technical masterpieces. In order for buildings and other structures to remain reliable, a substance is needed that will not allow the constituent parts to disintegrate separately.

Cement is a material that serves to bind building elements. Its application is great in the modern world. It is used in various fields of human activity, and the fate of all structures depends on it.

History of occurrence

Began to be used in ancient times. At first it was unbaked clay. Due to its ease of obtaining and prevalence, it was used everywhere. But due to its low viscosity and stability, clay gave way to heat-treated materials.

In Egypt, the first high-quality building materials were obtained. This is lime and gypsum. They had the ability to harden in air, due to which they were widely used. These building materials met the requirements until navigation began to develop. A new substance was needed that would resist the action of water.

In the 18th century, a material was invented - romancement. This is a product that could harden both in water and in air. But the increased development of industry required better materials and binding properties. In the 19th century, a new binding agent was invented. It is called Portland cement. This material is still used today. With the development of mankind, new requirements are imposed on binders. Each industry uses its own brand, which has the necessary properties.

Compound

Cement is the main component of the construction industry. The main components in it are clay and limestone. They are mixed together and subjected to heat treatment. Then the resulting mass is ground to a powder state. The gray fine mixture is the cement. If it is mixed with water, then the mass will eventually become like a stone. The main feature is the ability to harden in air and resist moisture.

Obtaining a cement mortar

In order for the building mass to be of the required quality, the composition must include at least 25% liquid. Changing the ratio in any direction leads to a decrease in the operational properties of the solution, as well as its quality. Setting occurs 60 minutes after adding water, and after 12 hours the mixture loses its elasticity. It all depends on the air temperature. The higher it is, the faster the mass will harden.

To obtain a solution, sand is needed, to which cement is added. The resulting mixture is thoroughly mixed and filled with water. Depending on the work performed, the solution can be ordinary or enriched. The first consists of proportions 1:5, and the second - 1:2.

Types and production of cement

At the moment, many varieties of binder are being produced. Each has its own degree of hardness, which is indicated in the brand.

The main types include:

• Portland cement (silicate). It is the foundation of all kinds. Any brand uses it as a foundation. The difference is the amount and composition of additives that give cement the necessary properties. The powder itself has a gray-green color. When liquid is added, it hardens and hardens. It is not used separately in construction, but goes as a basis for creating
• The plasticized composition reduces costs, has the ability to remove the mobility of the solution and perfectly resists the effects of cold.
• Slag cement. This is the result of crushing the clinker, and adding active additives. It is used in construction for the preparation of mortars and concrete.

• Aluminous. It has high activity, setting speed (45 minutes) and hardening (complete occurs after 10 hours). Also a distinctive property is increased resistance to moisture.
• Acid resistant. It is formed as a result of mixing quartz sand and sodium silicofluoride. To prepare the solution, sodium is added. The advantage of such cement is its resistance to acids. The disadvantage is a short service life.
• Color. Formed by mixing Portland cement and pigments. Unusual color is used for decorative work.

Cement production consists of 4 stages:

• Extraction of raw materials and their preparation.
• Roasting and production of clinker.
• Grinding to powder.
• Addition of necessary impurities.

Methods for the production of cement

There are 3 methods that depend on the preparation of raw materials for heat treatment:

• Wet. With this method, the required amount of liquid is present at all stages of cement production. It is used in situations where the main components cannot participate in the technological process without the use of water. This is chalk with a high moisture content, plastic clay or limestone.

• Dry. All stages of cement production are carried out with materials containing a minimum amount of water.
• Combined. Cement production includes both wet and dry methods. The initial cement mixture is made with water, and then it is filtered as much as possible on special equipment.

Concrete

It is a building material that is formed by mixing cement, filler, liquid and necessary additives. In other words, it is a hardened mixture that includes crushed stone, sand, water and cement. Concrete differs from mortar in its composition and size of the filler.

Classification

Depending on which bonding material is used, concrete can be:

• Cement. The most common type in construction. The basis is Portland cement, as well as its varieties.
• Gypsum. Possesses the increased durability. Used as a binder
• Polymeric. The basis is Suitable for work on horizontal and vertical surfaces. It is an excellent material for finishing and landscaping.
• Silicate. The binder is lime and siliceous substances. By its properties it is very similar to cement and is used in the production of reinforced concrete structures.

Depending on the purpose, concrete can be:

• Ordinary. Used in industrial and civil construction.
• Special. It has found its application in hydraulic structures, as well as in road, insulating and decorative works.
• Special Purpose. resistant to chemical, thermal and other specific influences.

cement cost

Manufacturers produce products packaged by weight. The weight of bags of cement is 35, 42, 26, and also 50 kg. It is best to buy the last option. It is the most suitable for loading and saves on packaging. Depending on the object on which the repair work will be carried out, cement of various grades is used, which has its own cost. When paying, each bag of cement is taken into account. Its price is fixed and may fluctuate depending on the requirements of the seller.

Before you start calculating cash costs, you need to decide on one more nuance. Sometimes you can see an ad that shows a price below the standard. You should not fall into such a trap. In such cases, expensive cement is diluted with cheaper one. Winning a few rubles, you will lose the quality of the building material.

Take one 50 kg bag of cement. The price of the M400D0 brand will be 220 rubles. The cost of others may vary, but on average it is:

• M400D20 - 240 rubles.
• M500D0 - 280 rubles.
• M500D20 - 240 rubles.

If you need to use only a couple of bags of cement, then it is most profitable to buy them at the nearest building materials store. And if you need a large number, then you should contact the manufacturer.

Consumption of cement

Before performing any construction work, the question arises of how much cement is needed and what consistency the solution should be. Ideally, strength should be maintained and the proportionality of the components should not be exceeded.

When responsible and serious work is ahead, it is unacceptable to mix cement and sand “by eye”. If you do not spare the binder material, then with large volumes it will cost huge amounts of money.

So how much cement is needed for the work being done? Building codes (SNiP) will help answer. It takes into account all the factors that affect the production of the mixture. Focusing on the brand of the composition and taking into account all the factors, you can clearly find out the cement consumption rate per 1 cubic meter of mortar.

The main feature that many developers do not take into account is that the cement is distributed in the voids between the sand particles. Remember that the composition has activity. If stored indoors for a long time, the 500 grade will become 400 after a few months. Therefore, when buying, you should always ask for a certificate with the date of issue.

Cement. Classification and marking.

That's what you can't do without at any construction site, so it's without cement. It does not matter what kind of house is being built: wooden or brick. The difference is only in its quantity. Every home needs a foundation. And in brick, in addition, he goes to masonry. With block construction, whole rooms are cast from it. What about road construction? And protection from the elements of the sea? What about mudflow diversion? What about bridges and dams across turbulent rivers? This building material has been gained by suffering through experience over the course of centuries, therefore it is reliable and has such significance.

background

As soon as a person began to build a dwelling out of stone, a means was immediately needed that would bind these stones. At first it was just clay. But such buildings did not differ in durability, and outwardly the building looked unpresentable. Then the binding properties of lime were noticed. First, the ancient Greeks and Romans discovered this, and the Romans discovered that when pozzolana (volcanic ash) and trass (hardened volcanic ash) are added to lime, the dried masonry becomes almost monolithic. In Rus' from clayey limestonesgray lime was obtained, seizing in damp and wet masonry. In practice, both Rome and Rus' almost approached the production of cement experimentally: both clays and pozzolana contained oxides of iron and aluminum, which, as a result of exposure to water and lime, underwent a hydration process. Then for a long time there were no changes in the composition of the binder (only fillers in solutions changed). And more recently in 1822- 1824 .G. almost simultaneously, the Russian Cheliev and the Scot Aspind received building mixtures similar in composition to modern cements. Andthe Scot thought of obtaining clinker and producing cement from it. The name "Portland cement" also came from England, since the concrete from the Scot cement both in color and strength resembled a stone mined in the mountains near the city of Portland.

What is cement?

By itself in nature, it does not form anywhere. And, thank God, otherwise we would not see sand and grass, we would walk on concrete. This is an artificial building material that, when mixed with water, forms an astringent plastic mass. Over time, the mass hardens and becomes a stone-like body, a monolith. What distinguishes cement from other binders is that it gains strength and solidity.in high humidity conditions and even under water. If you take air lime or gypsum as a binder, then they harden only in air. The reason is that in concrete, the cement hardens not so much due to the evaporation of water, but because the water reacts with the cement. In this case, only solid or crystalline substances are formed and heat is released. Most likely, this is why the process of mixing cement and water is called a shutter, and not a dissolution. The formation of a monolithic mass occurs as a result of cement hydration. Therefore, if the concrete is allowed to dry quickly in the sun, it will "tear", that is, it will crack and its destruction will begin. To prevent this from happening, it is wetted until the concrete is completely hardened.

Cement production

First you need to prepare raw materials. The raw material is limestone. The best limestones for cement productionThese are marl, chalk and calcareous tuffs. Dolomites and gypsum, although they are limestones, degrade the quality of the cement. That is, the best cement is obtained from finely porous limestones with no silicon inclusions. Limestone is crushed and thoroughly mixed with clay. In the resulting mixture of clay, about a quarter, the rest is limestone. This composition enters a rotary kiln with a diameter of 2 to 7 meters and about 200 meters long. In the kiln, 1450°C is the "sintering temperature", at which time the clay and limestone particles melt and diffuse into each other. The composition leaves the kiln after 2-4 hours in the form of sintered lumps of various sizes, this is the so-called cement clinker. Next, the clinker is crushed to particles of 1-100 microns. At the same time, up to 6% gypsum is added, this is necessary to prevent the process of cement setting from moisture in the air. Why is cement in such a "rush" to set from atmospheric moisture? Yes, it's just that the sticking surface after grinding is very large: the surface area of ​​​​particles of just one gram reaches 5000 cm2. Are other mineral supplements added? Naturally, after all, cement is needed in the foundation, and for masonry, and for floors, for example, water-repellent or quickly hardening cement is needed. To obtain different properties, a different composition is needed, so mineral additives are designed to provide certain properties.

Classification of cements

There is no unified and comprehensive classification of cement, similar to the periodic system of Mendeleev or the classification of the plant world of Carl Linnaeus. Therefore, there are several classifications, each of which takes into account some separate category of features.

For example, there is classification of cement division by clinker, which is the basis of their production:

• - Portland cement clinker;
• - high alumina and alumina clinker;
• - sulfate ferritic clinker;
• - sulfate aluminate clinker.

By appointment cements subdivided into:

• - special;
• - general construction.

Some classifications are based on the material composition. Then cements are subdivided as follows:

• - cements with mineral additives;
• - non-additive cements.

There is a classification that takes into account compressive strength:

• - cements, where strength is not taken into account;
• - cements with strength M600, M550, M500, M400, M300, M200.

A couple of classifications generally take into account time periods. One, taking into account the speed of hardening, divides cements into:

• - normally hardening;
• - fast hardening.

Another takes into account the setting time:

• - fast-setting (up to 45 min);
• - normally setting (45 min-2 hours);
• - slow-setting (more than 2 hours).

Cement marking

The determination of the brand of cement is based on the determination of its strength. How is it defined? Cement is thoroughly mixed with sand in a ratio of 1:3. The finished mixture is closed with water. Water is taken in an amount of 40% by weight of cement. Cubes or parallelepipeds are molded from the resulting plastic mass. To correctly determine the strength, such a workpiece is kept in water for28 days. Then these pieces of concrete are pressure tested for bending and compression. Most often, to check the compressive strength, take the halves formed as a result of a break from a bending test. And, attention! The amount of pressure that was needed to crush the workpiece is the brand of cement. Let's say that it took a pressure of 500 kg / cm 2 . So this is cement with a brand of 500.

Now let's deal with the markings written, for example, on the bag. The inscription is MPTs400-D20. "M" means that structures using this cement will be frost-resistant, the letters "PC" mean that it is Portland cement, the number 400 is a brand that means compressive strength, "D" is the presence of organic additives, and the number after it shows the percentage these additives. Thus, we have a bag with frost-resistant Portland cement grade 400 with 20% organic additives.

Varieties of cement

At the head it is necessary to put high-quality Portland cement, which does not even contain mineral additives. Next come cements containing mineral additives to change properties. The next group includes cements containing organic additives (usually resins). Slag cement is also distinguished, from which massive concrete elements of the building are made. Additional letters on the marking can tell a lot about the varieties of cement.

1. 1. B. Fast hardening, intended for repair work.
2. 2. BC. White cement for finishing and sculptural works.
3. 3. PPC. Pozzolanic cement with finely ground silica. The main advantage is reduced heat dissipation. Because of this, the upper and inner layers give off heat evenly, which means that the concrete will not crack.
4. 4. SC. Sulfate-resistant cement with protection against the destruction of concrete by salts. Therefore, it is perfect for hydraulic structures.
5. 5. shopping center. Grouting cement for plugging gas and oil wells.
6. 6. ShTs. Slag cement produced without a clinker base.
7. 7. CC. Colored cement obtained by the introduction of coloring pigments.
8. 8. PL means that plasticizers are used, HF - hydrophobic additives, due to which the effect of non-wetting, water repellency appears.

In accordance with generally accepted technology, if the mortar or concrete is made in accordance with the rules and proportions, immediately after pouring into a mold, formwork or on the surface, it begins to harden. However, its strength characteristics do not increase immediately, but over a certain period of time.

During this period, even if visually the mortar or concrete looks solid, a significant load cannot be applied to them - the material may crack and collapse.

In this regard, novice builders are interested in the question of how much cement (concrete or mortar) dries, as well as what factors affect the slowdown or acceleration of this process.

Stages of hardening of the cement mixture

In general, a 30-day exposure of a freshly poured structure will be sufficient to continue construction work. In some cases, when pouring powerful foundations for buildings, structures or industrial equipment, this period is increased to 90 days.

For small “domestic” construction - pouring floor screed, laying ceramic tiles, arranging a concrete blind area or path and other similar work, you can walk and move objects on the surface after 72 hours from the moment the mortar or concrete was laid.

In this case, the material goes through two stages of hardening: setting and actual hardening.

• grasping. This is a fairly fast process - no more than 24 hours from the moment the mixture was prepared. The main factor influencing the setting speed is the ambient temperature.

In the warm season, when the air temperature is in the range of 20-22 degrees Celsius, the mortar (concrete) begins to “set” approximately 2 hours after mixing. If the air temperature fluctuates around 0 degrees, this process can drag on for 20 hours.

At the same time, the material retains “mobility” all this time, and if at this time you start to perform any actions with it, the “setting” stage can be significantly delayed in time.

• hardening. According to building codes and instructions, the mortar (concrete) hardens within 30 days after the structure is poured.

However, in this case, not complete curing is implied, but curing to such a value at which the next stage of construction work can begin. Full hardening occurs within one or even several years.

It should be noted that the indicated periods are valid while maintaining the optimum ambient temperature and humidity in accordance with the instructions. Also, in order for the set mortar or concrete to gain its strength evenly and not crack, its surface must be protected from direct sunlight (usually with plastic wrap), on very hot days, fill in the morning or evening hours, and during the day sprinkle the surface with water in within 72 hours.